Cellulosic biomass such as bagasse, rice straws, chaff, waste mushroom beds, compost, and woodchips is attracting attention as energy resources that do not threaten food production and as raw materials for chemical industry. In particular, a technique for efficiently saccharizing fermentation feedstock is much needed to convert cellulosic biomass into fuel ethanol.
However, saccharizing technique applicable to cellulosic biomass is much more complicated than the one applicable to starch because cellulose, a major component of cellulosic biomass, is macromolecular polysaccharide having a firm and persistent crystal structure.
There are three ways of saccharizing cellulosic biomass, namely physical, chemical, and enzymatic saccharification methods.
Physical saccharification includes processes using a ball mill or oscillating mill, and those performing steam explosion or pressurized hot-water treatment. However, since physical treatment requires enormous energy, it is used as pretreatment for chemical and enzymatic saccharification in many cases.
Chemical saccharification uses alkali or acid. Acid saccharification has often been used from long ago. Acid saccharification includes concentrated sulfuric acid saccharification and two-step saccharification using dilute sulfuric acid. Since both methods use sulfuric acid, waste treatment and reduction in environmental load are necessary, and there is a limitation in reducing cost and improving energy conversion efficiency.
Compared with acid saccharification, enzymatic saccharification bears less burden of recovering and treating waste liquid, and cost for building chemical-resistant facilities can be saved. In addition, saccharide yield is high because excess decomposition does not occur, and that is why the method has been in practical use as enzymatic saccharification of biomass high in starch content. However, since cellulose has a complicated crystal structure with crystalline cellulose surrounded by hemicellulose and lignin as described above, enzymatic saccharification of cellulosic biomass is extremely difficult compared with starch biomass. Consequently, pretreatment such as breaking crystal structure by physical or chemical pretreatment is currently performed, or a large amount of hemicellulase or cellulase are used in combination before enzymatic saccharification treatment is performed.
Since hemicellulase and cellulase derived from Trichoderma reesei, which is an aerobic filamentous fungus, are used industrially as cellulose diastatic enzymes, study of Trichoderma fungi has been conducted vigorously (JP2007-319040A).
It has recently been found that a type of anaerobic microorganism produces cellulosome, an enzymatic complex capable of decomposing cellulose quite efficiently.
Cellulosome has a protein-based structure with a number of macromolecular-polysaccharide-degrading enzymes bonded together. As a result of acting on cellulose in cooperation, these enzymes exhibit extremely high macromolecular-polysaccharide-degrading activity (Mocrobiol Mol Biol Rev. 2005 March; 69 (1); 124-54 and Biochem J. 2004 Sep. 15; 382 (Pt3): 769-81).